1. Field of the Invention
The present invention relates to a detector that detects in real time a position of a lane sign drawn on a road surface, for example, to determine a boundary of lanes based on an image obtained via a vehicle-mounted camera. The lane boundary detector according to the present invention can be utilized as a useful apparatus for a vehicle driving support system, for example, which executes an operation to automatically keep a vehicle inside the lane.
2. Description of the Related Art
Examples of conventionally known detectors that detect in real time the position of the lane boundary indicated by the lane sign drawn on the road surface can be seen, for example, in Japanese Patent Laid-Open No. H11-232467 and Japanese Patent Laid-Open No. 2002-312797.
The detector disclosed in Japanese Patent Laid-Open No. H11-232467 examines a plurality of road surface images which are periodically captured, and analyzes characteristics of a temporal change in a relative positional relation between two white lines that indicate the lane boundaries, to determine whether a main road has a branch road or not.
The detector disclosed in Japanese Patent Laid-Open No. 2002-312797 first determines which white line that indicates the lane boundary is to be eventually adopted, and then determines whether the selected white line has a branch point or not.
The detector according to Japanese Patent Laid-Open No. H11-232467, however, does not provide a desirable result of detection in real time since the detector determines whether the branch exists or not based on a plurality of or many road surface images.
On the other hand, it is difficult for the detector according to Japanese Patent Laid-Open No. 2002-312797 to achieve a simple, flexible, constant and speedy selection of the proper white line indicating the lane boundary based on the result of the branch presence determination, since the detector first determines the white line to be examined and then determines whether there is a white line corresponding to a branch road in the vicinity of, i.e., outside the selected white line.
On highways, vehicles are generally supposed to drive at the speed of 20 to 30 meters per second at the slowest. Hence, when the road surface images are input to the detector in a cycle of 100 milliseconds, for example, the vehicle would move forward approximately three meters while the detector detects and determines a branch point in one image input cycle behind. In the vicinity of lane branch point, a road surface image corresponding to the actual road surface of approximately three meters' length, particularly of an area around the vehicle and more specifically of areas in front of and right behind the vehicle, provides various important pieces of information that allow the correct branch determination. Hence, even the time lag of one image input cycle cannot be overlooked. Thus, the technique according to Japanese Patent Laid-Open No. 2002-312797 does not satisfactorily solve the problem of time lag in determination process.
Though it would be possible to mount a high performance camera on the vehicle to set a shorter image pick-up cycle, such solution would increase the manufacturing coat.
In addition, the conventional lane boundary detectors cannot readily distinguish different types of lanes. For example, when the lane boundary is indicated by a combination of lines such as a plurality of white lines or dotted lines, the lane boundary cannot be readily separated, i.e., the contour thereof and the correct boundary cannot be distinguished depending on various factors such as the position of vehicle-mounted camera, the resolution of image pick-up, the driving speed of the vehicle, and the clarity of the lane indication. Hence, the conventional lane boundary detector is not immune to detection error when determination is made based on ill-defined road surface image, particularly when a white line candidate is selected in the vicinity of the branch point of a branch road.